However, existing statistical-CSI based contributions mainly characterized the systems performance under the assumption that perfect instantaneous overall CSI is available at the base station (BS). In practice, channel estimation error is inevitable.
Only a few contributions have taken imperfect CSI into consideration when configuring the RIS based on statistical CSI[1]}, [2]}, [3]}. Specifically, the impact of imperfect CSI on multi-RIS single-user systems was evaluated in [1]}, where the channel estimation is performed for each individual channel.
Different from [1]}, the authors in [2]}, [3]} investigated the system performance when considering the cascaded channel estimation instead of individual channels.
However, the contributions[2]}, [3]} only considered a simple channel model where the RIS-BS link is fully line-of-sight (LoS) while RIS-users links are fully non-LoS (NLoS). Although this model is mathematically tractable, it cannot reveal the fundamental impact of the scatter richness of the propagation environment on RIS-aided systems. With a variety of richness of scatter, the quality of channel estimation and the system capacity performance in RIS-aided systems will exhibit wide differences.
